Various processes and apparatuses for the dyeing of yarns are already known.
Thus, for example, a hank-dyeing process is known in which yarn delivered from the spinning mill on a bobbin and rewound onto a conical bobbin is unwound on a hank-reeling machine, in order to obtain yarn in the form of hanks, with further handling taking place with transport of the bobbins. Each hank is then attached manually at different points, and the two ends of each hank are tied up with one another. The resulting hanks are then placed on dyeing beams, attached to a carriage and transferred to a dyeing apparatus. After dyeing, the hanks are washed, their excess water is removed in a hydroextractor, and the hanks are then dried either in a hot air through-circulation oven or by passing them continuously through a drying tunnel. After this drying, the hanks are placed again on a winding device to be rewound onto conical bobbins usable in knitting or weaving.
Such a dyeing process, which gives acceptable results as regards the inter-fibre penetration of the dyeing liquor, necessitates many handling operations, resulting in relatively long process times which are stressful to the material and in high production costs.
Package-dyeing processes are also known, in which yarn delivered from the spinning mill on a bobbin is rewound onto a conical or a cylindrical bobbin, the borders of the bobbin being optionally rounded by pressing, in order to allow better subsequent circulation of a dyeing liquor into the inside of the bobbin.
According to this embodiment, another handling operation consists in stacking bobbins on perforated columns of the material support of a dyeing apparatus, after which this material support is placed in an autoclave and a dyeing liquor is injected under pressure into the autoclave by means of a pump. After a predetermined period, the dyeing liquor is removed, and washwater is injected into the autoclave. The material support is then removed from the autoclave, and the bobbins are placed in a hydroextractor to remove excess water.
After this operation, the bobbins are dried batchwise in a drying cabinet or continuously in a high-frequency oven. After drying, the yarn is rewound.
The excess water can also be removed by means of apparatuses referred to as high-speed dryers, in which the unit comprising material support and bobbins is drained by means of a vacuum pump and is then pressurized in an autoclave by means of air or steam. In such a case, additional drying is no longer necessary.
Like the previous process, such a package-dyeing process requires many operator interventions and extends over a relatively long period, as a result of which the costs of this process are relatively high.
On the other hand, these traditional package- or hank-dyeing processes are not capable of producing special yarns, such as chine yarns, "space" yarns, single- or multicolour speckled yarns, or yarns with partial impregnation of the fibres, for example of the washed-out type, of the ring-dyed denim type or of the stone washed denim type.
Moreover, when dyeing cellulose fibres, plant fibres, such as cotton, linen etc., or chemical fibres, such as the acetate and viscose fibres, the known dyeing processes involve a high consumption of water, i.e. in the order of 200 to 300 liters per kilogram of dyed yarn.
Such a consumption of water has not only the disadvantage of the cost of the consumed water, but also that of the treatment of the water enriched with dyes and the residual chemical auxiliaries before it is discharged.
However, in the case of the dyes used for cellulose fibres, the customary reagents must also be taken into consideration which, when used for preparing the dyeing liquor, require a large amount of added salt, namely in the order of 80 to 100 g per liter of dyeing liquor, of which only 60 to 70% of the mixture are fixed during the dyeing process and the remaining 30 to 40% of the mixture are discharged during washing and represent a major pollution source, requiring a particularly thorough treatment of the wastewater.
Furthermore, when dyeing in the form of packages and, to a lesser extent, in the form of hanks, the multiple unwinding and rewinding operations of the yarn have adverse effects on the quality of the yarn product, including, on the one hand, a loss of weight and, on the other, a loss of tenacity and elongation of the yarn.
Apparatuses are also known in which processes for the continuous dyeing of textile yarns are carried out by continuous impregnation of moving yarns, which are subsequently transferred to a conveying means, which takes the yarns through chambers containing steam or hot air under atmospheric or elevated pressure. Such a process, called thermofixing, enables the dyes to be fixed on the fibres constituting the yarn.
Such apparatuses make it possible in particular to dye yarns based on polyamide, polyester, polypropylene and acrylic fibres and also wool and fibre blends, with a substantially reduced consumption of energy and water compared with the treatment processes in the form of packages or hanks.
These continuous treatment processes only make use of acid dyes, disperse dyes or basic dyes.